U.S. Pat. No. 6,259,763 discloses a high-resolution x-ray imaging crystal spectrometer to record spatially resolved impurity line spectra emitted from tokamaks and other extended plasma sources, used in magnetic confinement nuclear fusion energy research, for Doppler measurements of ion temperature and toroidal plasma rotation velocity profiles. The spectrometer concept is based on the Johann configuration, but the typically used cylindrically bent crystal and one-dimensional, position-sensitive detector are replaced by a spherically bent crystal and a two-dimensional, position sensitive detector, whereby spatial resolution or 1d-imaging is obtained in a direction perpendicular to the main diffraction plane. The imaging properties of this spectrometer, which are schematically depicted in FIG. 1, are determined by the astigmatism of a spherical reflector, such as a crystal or minor, 10, due to the fact that the images formed by the sagittal and meridional rays 12 and 14 emanating from a point source on the minor's Rowland circle 16 are spatially separated and mutually perpendicular lines at Fs and Fm. Thus, by reversing the rays in FIG. 1, we see that photons, which seem to emanate from the sagittal line image Fs, are being focused to a point on the Rowland circle. The possibility of obtaining spatially resolved spectra follows immediately from the rotational symmetry of the ray pattern about the normal 00′ of the spherically bent crystal 10, since by a rotation about this normal the point source (or point image) S and the associated sagittal line image (or line source) Fs move on a cone in opposite directions above and below the main diffraction plane, so that different points on the detector correspond to different locations in the plasma. On tokamaks, the preferred experimental arrangement is such that the main diffraction plane coincides with the horizontal mid-plane, so that the sagittal line image (line source) Fs is parallel to the toroidal magnetic field, along which the electron density, electron temperature, and therefore the x-ray emissivity are uniform. Spatial resolution and 1d-imaging of tokamak plasmas is thus obtained perpendicular to the toroidal magnetic field. This 1d-imaging scheme has been successfully used for the diagnosis of (extended) tokamak plasmas on NSTX and Alcator C-Mod, where, with the proper experimental arrangement, the astigmatism of a spherical mirror or crystal is not a matter of concern. This imaging scheme is, however, not readily applicable to other plasma sources, where a direction of symmetry or uniform x-ray emission is not provided. In such cases, other optical systems, with near paraxial rays, i.e., restricted optical apertures and small angles of incidence, are most commonly used to avoid astigmatism. Although, variants of the optical scheme shown in FIG. 1 are also used in the diagnosis of laser-produced plasmas and Z-pinch plasmas. But the images obtained with a single spherically bent crystal from such sources are, in principle, not sharp and flawed by astigmatic errors even if the highest, still practicable, Bragg angles of 80° are used, so that these images are not fully satisfactory for a detailed data analysis.